MXPA97003322A - Method and apparatus for evaluating in formacuantitative the tension of the winches of the an electron alternator - Google Patents

Abstract

The present invention relates to a method for quantitatively evaluating the tension of the stator wedges of an alternator comprising a rotor and a stator, which together define an air gap of a given width, the stator has radial grooves, with outlets having an open communication with the air gap, in each of the slots, at least one of the stator coils is fixed by an elastic retention mechanism which is in contact with wedges which are slidably inserted in a symmetric set of notches provided for this purpose in the stator at the outlet of each slot, this method is characterized by comprising the following steps: a. inserting in the air gap a sensor having a movable element, capable of being displaced in the direction transverse to this air gap by the effect of a pressure exerted by a fluid; position the sensor in front of the wedge of the stator whose voltage will be evaluated, in such a way that this mobile element faces the wedge of the stator; exert a pressure on the fluid to move the moving element of the sensor and make this element contact first and then press the wedge of the stator, while the sensor is backed up against the rotor; measure the pressure exerted in this way, this pressure being proportional to the force applied to the elastic retention mechanism that is located behind the wedge of the stator e. measure the displacement of the stator wedge when the pressure is exerted, and f. determine the tension of the stator wedge, using force and displacement measured in this way, in which: g. because the sensor used has a body formed by a flat and thin plate, with a thickness slightly less than the width of the air gap, this plate has a surface with a recess that acts as a cylinder in which a piston is inserted that operates as the aforementioned mobile element, this piston can be operated remotely, injecting the pressurized fluid into the cylinder, and h. because the measurement of wedge displacement is achieved by measuring the displacement of the piston in its cylinder, by means of at least one elastic blade having a first end firmly fixed to the surface of the sensor, a second end mounted on the piston, so that can be displaced with this, and a central portion to which at least one strain gauge is attached gives a signal proportional to the displacement of the piston and therefore, to the displacement of the wedge of the stator when this is pressed

Description

METHOD AND APPARATUS FOR QUANTITATIVELY ASSESSING THE TENSION OF THE STATOR WEDGES OF AN ELECTRICAL ALTERNATOR BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for quantitatively evaluating the tension of the stator wedges of a large alternator. The invention also relates to an apparatus for carrying out this method. In the following specification, the term "alternator", as used, designates any class of hydroalternators and turboalternators, with a vertical or horizontal axis. 2. Description of the Prior Art As is known, an alternator comprises a rotor and a stator that define an air gap of a given width between them. In practice the stator generally consists of superimposed metal plates, in which there are radial grooves with outlets that open within the air gap. In each of these slots, an elastic retaining mechanism supports at least one and preferably two stator coils; this holding mechanism may consist of a rubber insulating bead or a spring for fluctuations which, in turn, may consist of a long strip of composite material which is longitudinally or transversely corrugated. The elastic retention mechanism is in contact, directly or by means of a packaging material and / or angles, with a set of small wedges of a stator, a few centimeters long, which are slidably inserted and fixed in a set of symmetrical notches disposed for that purpose in the metal plates of the stator, at the outlet of the corresponding slot. In large alternators, such as those powered by the turbines of a dam, there is a large number of these wedges, preferably made of a composite material of the glass fiber type. Indeed, in large alternators, the stator can have up to 500 slots along its periphery, and each of them can be several meters high. It is known that one of the main causes of forced interruptions of the use of the alternators used in hydroelectric power plants is due to defects in the winding of the stator coils, which are caused in turn by the vibrations of the coils of the stator. The vibrations can be caused by improper mounting of the wedges in their notches or if the elastic retention mechanism mounted between these wedges and the stator coils is no longer tense with use. In these cases, there is in fact premature wear and the consequent risk of short circuits. Therefore, a standard procedure is to check at regular intervals whether the stator coils are properly shielded. The method currently used to evaluate the tension of the stator wedges of an alternator, that is, to detect if the stator coils are properly shifted in place, consists of hammering each of the stator wedges and detecting the sound produced if the pressure exerted by the elastic retention mechanism located behind the wedges keeps them correctly in their notches. This method is simple and effective but has two main disadvantages. First the rotor must be partially or completely disassembled, so that the operator can make the necessary strokes. This can only be done on some important occasions. Second, this method is purely subjective, because the determination of the degree of "tension" of the wedges according to the sound produced by them is completely at the discretion of the operator who, although well trained to do so, can make mistakes. To try to overcome these disadvantages, ELIN-UNION A.G. of Austria proposed to hit the wedges of the stator without removing the rotor, with a micromartillo that can be inserted in an air gap whose width is greater than 10 mm. The sound produced by the beats is registered with a microphone and can therefore be evaluated by an operator using headphones or by a computerized electronic sound analysis system. A similar method is also proposed in US Patent No. 4,970,890, conferred on November 20, 1990 to ESTINGHOUSE ELECTRIC CORPORATION. In the German patent application No. DE-A-3,438,468 which was opened, published on June 20, 1985, ELIN-UNION A.G. He also presented another method to evaluate the tension of the stator wedges of an alternator without having to remove the rotor, which consists of checking the tension of the stator wedges of the elastic retention mechanism that fixes the wedges in place, with a sensor which has a thickness less than the width of the air gap; the sensor comprises a movable element that can be operated remotely by wedge-shaped mechanisms located at the ends of a piston, operated in turn by a hydraulic fluid. During use, the sensor is inserted in the air gap and placed in front of the wedge where the evaluation will be carried out, so that the moving element of the sensor faces the wedge. Then the piston is actuated and the pressure "P" of the injected fluid is measured, at the same time as the longitudinal displacement of the piston which, thanks to the wedge-shaped mechanisms, causes a simultaneous transverse displacement of the sensor, whose value "L "It can be easily calculated. The curve indicating the pressure "P" as a function of the displacement "L" obtained in this way makes it possible to evaluate the tension of the stator wedges of the elastic retention mechanism. Although this method is very effective, it can only be used, according to its manufacturers, in air intervals whose widths are greater than 20 mm. In addition, this method is difficult to apply in large alternators because it does not seem to include mechanisms to position the sensor and its moving element correctly in front of each of the wedges to be verified. A similar method was proposed in U.S. Pat. # 5,012,684, conferred on May 7, 1991 to ESTING HOUSE ELECTRIC CORPORATION. This patent describes a method to evaluate the tension of the stator wedges of an alternator, using a sensor specially designed to measure the elasticity of the rubber insulating bead or the spring for fluctuations that fixes the wedges, and measuring the displacement of This rubber insulating bead or spring for fluctuations as a function of the pressure exerted. In this particular case, the radial stator slots in which the stator coils are located must have two sets of notches: one for the stator wedges and the other in which the sensor can be inserted. This configuration does not allow the sensor to be operative, unless the alternator is of the type previously described. WESTING HOUSE ELECTRIC CORPORATION also proposed another type of sensor that can be inserted in the air gap of an alternator, to measure the deviation of the springs for fluctuations that fix the wedges of the stator. The sensor object of U.S. Pat. # 5,020,234, conferred on June 4, 1991, is very complicated from a structural point of view. In addition, although an additional set of notches is not required, the wedges must have perforations, because the sensor is designed to measure the deviation of the springs for fluctuations, by the contact of the same with a rod that must be aligned with the selected perforation. , using a television camera to do it. SUMMARY OF THE INVENTION A first object of the invention is to provide a method for quantitatively evaluating the tension of the stator wedges of an alternator; The main advantage of this method is that it does not have any of the limitations of the known methods already described. A second object of the invention is to provide an apparatus for carrying out this method. In particular, the first object of the invention is to provide a method for evaluating the tension of the stator wedges of an alternator having the following advantages, thanks to the use of a new sensor designed especially for this purpose, which is very simple, thin and effective: • uses very simple structural mechanisms; • can be carried out without having to remove or disassemble the alternator rotor; • can be carried out on any type of alternator, even when the width of the alternator air gap is as thin as 10 mm; • allows to obtain a quantitative evaluation of the percentage of equivalent compression of the elastic retention mechanism, in a simple, effective and reliable way; • can be automated. As in all the above-mentioned methods, the method of the invention will be used to quantitatively evaluate the tension of the stator wedges of an alternator comprising a rotor and a stator, which together define an air gap of a width dice; The stator has radial grooves, with outputs that have an open communication with the air gap. In each of the slots, an elastic retention mechanism, such as a rubber insulator bead or a spring for fluctuations, fastens at least one stator coil; this mechanism is in contact with wedges that are slidably inserted in a symmetrical set of notches provided for this purpose in the stator, at the outlet of each slot. As in the method described in the German patent application no. DE-A 3,438,368, which was opened, the method of the invention is of the type characterized by comprising the following steps: a. inserting in the air gap a sensor having a moving element, capable of being displaced in a direction transverse to this air gap by a fluid, subject to a pressure; b. position the sensor in front of the wedge of the stator whose voltage will be evaluated, in such a way that the mobile element of this sensor faces the wedge; c. exert a pressure on the fluid to move the moving element of the sensor and bring this element so that it is connected and then press against the wedge, while the sensor is backed up against the rotor; d. measuring the pressure exerted in this way, this pressure being proportional to the force applied to the elastic retention mechanism that is located behind the wedge of the stator; and. measure the displacement of the wedge of the stator when the pressure is exerted; and f. Determine the tension of the stator wedge, using the force and displacement measured in this way to do so. However, the method of the invention is distinguished from the known methods mentioned above: g. because the sensor used has a body formed by a flat and thin plate, with a thickness slightly less than the width of the air gap; this body has a surface with a recess that acts as a cylinder in which a piston is inserted that operates as the aforementioned moving element; This piston can be operated remotely, injecting the pressurized fluid into the cylinder; and h. because the measurement of displacement of the wedge is achieved by measuring the displacement of the piston in its cylinder, by means of at least one, and preferably two elastic sheets each having a first end firmly fixed to the surface of the sensor; a second end mounted on the piston, so that it can be moved with it; and a central portion to which at least one is attached, and preferably two or four strain gauges that give a signal proportional to the displacement of the piston and therefore, to the displacement of the wedge of the stator when it is pressed. From a practical point of view, it is possible to determine the tension of the wedges of the stator with a first method that can be carried out in two different w that is to say: 1. exerting a constant force and measuring the displacement obtained; or 2. determining a given displacement for the wedge of the stator and measuring the force required to achieve this displacement. Before applying this method, it can be very useful to calibrate the sensor correctly, measuring in the laboratory the force that will be exerted as a displacement function to be obtained, for the type of elastic retention mechanism that will be tested in different initial compression levels. In this way, it is possible to draw a calibration curve indicating the displacement produced by a given force "F", or the force required to obtain a given displacement "D", as a function of the initial compression level. The expression "initial compression level", as used in this specification, designates - Il ¬ the ratio of the actual compression of the elastic retention mechanism when it has just been installed, with the maximum compression that can be given to that mechanism, expressed as a percentage. In this way, in the case that the elastic retention mechanism is a spring, this initial compression level is the ratio, expressed as a percentage, of the actual spring travel (expressed in a unit of given length) when it has just been installed , with the total travel that spring has when it is completely compressed. The level of compression (or tension) of each wedge during the tests can then be derived directly from the measurement of the displacement produced by a given force "F" applied to the wedge, or from the measurement of the force required to move the wedge through a wedge. given "D" length, using any of the calibration curves to do so. This will be explained in detail below. The evaluation of the tension of the wedges of the stator can also be achieved by a second method that consists of evaluating the ordinate at the origin of the curves that indicate the force as a function of the displacement. This corresponds to the actual force applied by the elastic retention mechanism on the stator coils. In this case, the initial ordinate can be obtained by the computerized processing of the curves.

If necessary, it is possible to proceed to comparatively analyze the curves that indicate the force as a function of the displacement of new and used elastic retention mechanisms, respectively, to determine the mechanical state of the elastic retention mechanism that will be evaluated at the moment of the measurement. As mentioned before, the sensor used to carry out the invention has a miniature piston and a set of elastic sheets. The structure of this sensor allows it to have a thin thickness of 10 mm that facilitates its use in the air intervals of most existing alternators. The force is measured with a fluid pressure sensor and the displacement is measured with elastic blades that have strain gauges that directly measure their deflection, that is, the displacement of the piston. The apparatus that incorporates this sensor can be completed with a console that has a control, a module, a data acquisition module, an analysis module and a video screen. In addition, an automated positioning system can be included to automatically move and position the sensor from one wedge to the other and from one notch to the other. There may also be a camera to facilitate the positioning of the sensor when it is used manually, and to allow a visual inspection of the ventilation openings and the surface of the center of the stator, as well as to verify the piston, the wedges and the leaves. of the sensor when necessary. As mentioned before, the second object of the invention is to provide an apparatus specially designed to carry out the method already described. This apparatus will be described in detail below. The reading of the following non-limiting description of a preferred configuration of the invention, made with reference to the accompanying drawings, will allow a better understanding of the invention and its numerous advantages. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a side view, partially transverse, schematic, of an apparatus according to the invention, for evaluating the tension of the stator wedges; the apparatus is shown in the position of use, with the sensor inserted in the air gap of an alternator, in front of a wedge of the stator, whose percentage of compression will be determined. Figure 2 is a cross-sectional view taken along lines II-II of Figure 1. Figure 3 is a top aerial view of the apparatus of Figure 1. Figure 4a is a top aerial view of the apparatus sensor. illustrated in Figures 1 to 3.

Figure 4b is a cross-sectional view of the sensor, taken along line IV-IV of Figure 4a. Figure 5 is a block diagram of the control circuit of the apparatus. Figure 6 is a graphical representation showing the force applied by the sensor as a function of the displacement measured at different compression levels. Figure 7 is a graphic representation showing the displacement caused by a certain force, in different compression levels. Figure 8 is a graphical representation showing the force required to obtain the same predetermined displacement at different compression levels. DESCRIPTION OF THE PREFERRED MODE OF THE INVENTION As explained above, the method and apparatus of the invention are designed to be used in the quantitative evaluation of the tension of the stator wedges of an alternator comprising a rotor 1 and a stator 3, which together define an air gap 5 of a given width (see figures 1 and 2). The stator 3 has radial grooves 7 with outlets that open towards the air gap 5; in each of the slots, one or two stator coils 9 are held by an elastic retention mechanism which may consist of insulating beads of rubber or other similar material or in springs for fluctuations formed by a composite material, longitudinally or transversely corrugated. In the accompanying drawings and in this specification, the illustrated elastic retention mechanism consists exclusively of a longitudinally corrugated fluctuation spring 11 which is in contact with the stator wedges 13 of the trapezoidal cross-section, inserted in a set of notches 15, arranged symmetrically for that purpose in the stator, at the outlet of each slot 7. However, it will be understood that the invention is not limited to the same specific class of spring for fluctuations illustrated. In this way, in the illustrated configuration, the spring for fluctuations 11 consists of an elongate sheet of composite material, corrugated, which is in contact, directly or by means of a stretch of packing material 17 and a plurality of angles 19, with the coil of the stator 9 adjacent and with the wedges of the stator 13 which are also formed by a composite material. As can be seen more clearly in Figures 1 and 3, the apparatus of the invention comprises a sensor 21 sized such that it can be inserted into the air gap 5. The structure of this sensor is an essential element of the invention and will be described in detail below. The apparatus also comprises mechanisms for positioning the sensor in front of each wedge of the stator 13 where it is necessary to evaluate the voltage. These mechanisms preferably include a first mechanical system, adapted to move the sensor 21 along each radial slot 7, and a second mechanical system, adapted to move the sensor from that slot to one or another adjacent slot. As it is observed, the first of these two mechanical systems has a chain 23, with the links 24 that rotate around an axis only with respect to the others. The chain 23 is wound around a drum 25 and has a free end that can be extended by developing it in the air gap, and to which the sensor 21 is attached. The drum is operated, manually or electrically, by a motor 27 which can be connected to an appropriate control system, to move the sensor automatically along each radial slot 7, in such a way that the sensor 21 is positioned in front of each successive wedge of the stator 13, slidably inserted in the notches 15 of slot. The second of the aforementioned mechanical systems comprises a trolley 29 on which the first mechanical system is mounted, and other mechanisms, such as an auger 31 which can be operated by an electric motor 33 to move the trolley during transport. The motor 33 can also be connected to the control system 28, to ensure automatic movement of the device from a slot 7 to one or more other adjacent slots. These mechanical systems are simple and take up little space. They can be mounted in a box 35, transportable as a kit and designed to be easily installed with electromagnets on an alternator, just above the area of the air gap where the tension of the stator wedges will be valued. When this is possible, the box can advantageously be mounted on the rotor of the alternator so that it can rotate with it and can thus move along the entire periphery of the stator to be inspected. With respect to Figures 2 and 4, the sensor 21 of the invention comprises a body 37 consisting of a thin metal plate whose thickness "h" is constant and is chosen so that the sensor can be inserted in the air gap. The thickness "h" is preferably less than or equal to 10 mm. The thin body 37 is attached to the end of the last link 24 of the chain. It has a surface 39, - l formed by a recess 41 preferably cylindrical acting as a piston cylinder, in which a piston 43 is inserted in a movable manner. The piston has a head 45 of the same diameter as the recess 41 and a short coaxial rod 47, which has or does not have a tiltable tip 48, mounted on a cylindrical hinge 49 (see Figure 2). This inclinable tip 48 fits the surface of the wedge of the stator 13 and prevents the exerted pressure from applying a cantilever force on this wedge. The piston 43 is fixed in the recess 41 by a retaining ring 50, attached to the periphery of the recess by the screws 51. The piston 43 can be controlled remotely by a mechanism for injecting a fluid under pressure, preferably air, into the recess 41 of the body, behind the head 45 of the piston. This injection mechanism includes a source of compressed air (not shown), an air feeding tube 53 passing through the chain, and a passage 55 made in the body of the sensor that opens in recess 41. The object of this The injection mechanism is essentially to exert a pressure by means of the injected fluid, in such a way that the piston 43 moves in the direction transverse to the air gap 5 when the sensor 21 is inserted in the latter, so that the piston applies a pressure against the wedge of the stator 13 in front thereof, while the sensor is retracted against the rotor 1. Further, there is a retractor mechanism consisting of a plurality of small sheets of elastic 56 for moving the piston 43 back inside the cylinder, as soon as stops the injection of fluid. There are other mechanisms not illustrated, but well known per se, for measuring the pressure exerted in this manner, when the piston 43 presses against the wedge of the stator 13. It will be understood that the pressure exerted is proportional to the force applied to the spring for fluctuations 11. , behind the wedge of the stator 13. Likewise, there are other mechanisms for simultaneously measuring the displacement of the wedge of the stator 13, while exerting the pressure. These other mechanisms actually measure the displacement of the piston 43 in its cylinder 41. For this, these mechanisms comprise at least one, and preferably two elastic sheets 57; each of them has a first end firmly fixed with screws to the surface 39 of the sensor body 37, a second end which is in contact with the head 45 of the piston 43 to move with it, and a central portion. On at least one of these central portions, at least one or preferably two or four strain gauges 59 are attached, electrically connected to measuring devices (not shown) by a core assembly 61. When the piston is displaced, the blades 57 which they also act as a piston retractor mechanism, flex and cause a change in the resistance of the wire of each meter 59. The latter then emits a signal proportional to the displacement 43 and, therefore, to the displacement of the wedge of the stator 13 when is pressed. According to a preferred configuration of the invention, especially useful when the positioning of the sensor is done manually, a display system can be installed to allow the operator to locate each wedge of the stator, and then position the sensor 21 relative to that one. This system may comprise a microcamera or a fibroscope, such as those used for an endoscopy. In this case, the optical fiber 62 of the display system has an end acting as a lens, fixed to the sensor 21 in a cabin 63, provided for that purpose in the body 37 (see Figure 4), transverse to the piston 43 The end of the fiber 62 acting as a lens can be adjusted remotely, as is known, and, in front of it, there is a 45 ° mirror 65 incorporated into the cavity 63. A lighting mechanism can be conveniently incorporated 67 , in addition to those of the visualization system in the body 37 of the sensor, to facilitate the location of the stator wedge whose voltage will be evaluated using the fiber 61 and the mirror 65. The apparatus, in practice, once installed, can be used as follows: a. the sensor 21 is inserted in the air gap 5; b. the sensor is positioned in front of the wedge of the stator 13 where the tension must be evaluated, in such a way that the piston 43 faces the wedge of the stator; c. a variable constant or pressure is exerted by compressed air or any other fluid, to move the piston 43 of the sensor and bring it to be contacted first and then press against the wedge of the stator 13, while the sensor is retracted against the rotor 1; d. the pressure exerted in this way is measured; this pressure is proportional to the force applied to the spring for fluctuations 11 (or rubber isolation bead) by the wedge of the stator; and. at the same time, the displacement of the wedge of the stator is measured by the strain gauges 59, fixed to the elastic blades 57, when the pressure is being exerted; and f. the compression level (or tension) of the spring is determined by the force and displacement measured in this manner, using any of the above-mentioned methods and described in detail below. This last determination can be made by an electronic conditioning system and a computerized processing system 69 that has or does not have a video screen. Before carrying out steps (b) to (f), it is convenient to pre-calibrate the sensor 21, positioning it in the air gap and between the rigid surfaces of the rotor 1 and the stator 3, and measuring the displacement of the piston as a function of the work pressures that can be used. In step (f), the value of the displacement measured in this way can then be subtracted from the measured value of the same displacement when the pressure is exerted on the wedge. 13. To avoid having to perform this precalibration and, what is more important, to take into account any variation in the width of the air gap, the apparatus can also have a set of shoes 71, mounted on the free ends of the elastic sheets 73 that they are fixed in the cavities arranged for this purpose in the body 37; These shoes have deformation meters 74 connected to the sensor by appropriate wiring installations. These shoes 71 are positioned in such a way that they are in front of, are in contact with and slide on the surface of the center of the stator 3, on both sides of the groove where the measurement is made. Any variation in the width of the air gap will deflect the blades 73 and will be measured by the strain gauges 75 mounted on the central portion of these blades. Then, the measured values will be subtracted from the measured values of the displacement of the piston, to obtain the real value of the displacement of the wedge of the stator and of the spring for fluctuations. In practice, the determination step (f) can be carried out in three ways: After a correct precalibration, reference curves can be drawn in the laboratory, indicating the value of the force as a function of the displacement for the retention mechanism Elastic tested, in different initial compression levels. Then, a calibration curve can be drawn indicating the displacement value caused by a force F, as a function of the initial compression level of the elastic retention mechanism (see figure 6). This level of compression (or tension) of the stator wedge during the test can then be deduced directly from the displacement measurement caused by a given force F, applied to the stator wedge, using the calibration curve. See figure 7. In this way, according to this method, a constant pressure is exerted and the resulting displacement is measured. It is also possible to evaluate the tension of the wedges of a stator, defining a given length for the displacement and measuring the pressure that must be exerted to achieve this displacement (see figure 8). Finally, the tension of the stator wedges can be evaluated, evaluating the ordinate at the origin of the curves that indicate the force as a function of the displacement, which corresponds to the actual force applied by the spring on the stator coils. . In this case, a threshold value corresponding to an ordinate can be established which in turn corresponds to a required compression level, and then check whether the measured value of the force exerted reaches this threshold or not. You can get this ordered at the origin by the computerized processing of the curves. As mentioned before, this whole process can be automated. It is understood that modifications may be made to the apparatus described herein, which is illustrative only, without departing from the scope of the invention as defined in the following claims. In this way, for example, the apparatus of the invention could also have at least one separation plate 77 that can be fixed to the sensor body on its surface opposite the piston, so that the thickness of the sensor increases and makes it useful at intervals of wider air (see figure 1).

Claims (24)

1. NOVELTY OF THE INVENTION Having described the present invention, it is considered as a novelty and therefore the property described in the following claims is claimed as property. A method for quantitatively evaluating the tension of the stator wedges of an alternator comprising a rotor and a stator, which together define an air gap of a given width; the stator has radial grooves, with outputs that have an open communication, with the air gap; in each of the grooves, at least one of the coils of the stator is fixed by an elastic retaining mechanism which is in contact with wedges which are slidably inserted in a symmetrical set of notches provided for this purpose in the stator in the output of each slot; This method is characterized by comprising the following steps: a. inserting in the air gap a sensor having a moving element, capable of being displaced transversely to this air gap by the effect of a pressure exerted by a fluid; b. position the sensor in front of the wedge of the stator whose voltage will be evaluated, in such a way that this mobile element faces the wedge of the stator; c. exert a pressure on the fluid to move the moving element of the sensor and cause this element to contact first and then press the wedge of the stator, while the sensor is backed up against the rotor; d. measuring the pressure exerted in this way, this pressure being proportional to the force applied to the elastic retention mechanism that is located behind the wedge of the stator; and. measure the displacement of the wedge of the stator when the pressure is exerted; and f. determine the tension of the stator wedge, using force and displacement measured in this way; in which: g. because the sensor used has a body formed by a flat and thin plate, with a thickness slightly less than the width of the air gap; this plate has a surface with a recess that acts as a cylinder in which a piston is inserted that operates as the aforementioned moving element; This piston can be operated remotely, injecting the pressurized fluid into the cylinder; and h. because the measurement of wedge displacement is achieved by measuring the displacement of the piston in its cylinder, by means of at least one elastic blade having a first end firmly fixed to the surface of the sensor; a second end mounted on the piston, so that it can be moved with it; and a central portion to which at least one strain gauge is attached giving a signal proportional to the displacement of the piston and therefore, to the displacement of the wedge of the stator when it is pressed.
2. 2. The method of claim 1, characterized in that, before performing steps (b) a (f), the sensor is positioned in the air gap between the rigid surfaces of the rotor and the stator, and the value of the displacement of the piston corresponding to the working pressures is measured, and in which, in step (f) , the displacement value measured in this way is subtracted from the value of the same displacement measured when the pressure is exerted on the wedge of the stator. The method of claim 1, characterized in that when step (b) is completed, any verification in the width of the air gap is measured by at least one strain gauge mounted on an elastic sheet having an end fixed to the body of the sensor, and another free end with a shoe that connects to and slides on a rigid surface of the stator, and in which, step (f), the variation measured in this way is subtracted from the value of the measured displacement to obtain the real value of the displacement of the wedge of the stator when pressure is exerted on it. The method of claim 1, characterized in that step (b) is performed by mechanical means designed to move that sensor along each radial slot. 5. The method of claim 4, characterized in that the mechanical means are controlled to automatically move that sensor along each radial groove, so that the sensor is positioned in front of each successive wedge of the stator and slidably inserted in the grooves made in that radial groove. The method of claim 5, characterized in that the mechanical means are also designed to move the sensor from a given slot to at least one other adjacent slot. The method of claim 4, characterized in that the mechanical means are manually controlled, and in which the sensor has a fiber optic display system that allows the operator to locate each wedge of the stator and position the sensor relative to said wedge. stator 8. The method of claim 1, characterized in that the fluid used is compressed air. The method of claim 1, characterized in that step (f) consists in determining the compression ratio of the elastic retention mechanism, exerting a constant pressure during step (c) and measuring the displacement obtained in this way. The method of claim 1, characterized in that step (f) consists in determining the compression ratio of the elastic retention mechanism, establishing a given displacement and measuring the pressure required to achieve that displacement. The method of claim 1, characterized in that step (f) consists of determining the compression ratio of the elastic retention mechanism, establishing a threshold value corresponding to an ordinate of the curves indicating the force as a function of displacement , this ordinate corresponds to a percentage of equivalent compression required, and then verifying if the measured value of the force exerted reached this threshold value or not. 12. An apparatus for quantitatively evaluating the tension of the stator wedges of an alternator comprising a rotor and a stator, which together define an air gap of a given width; the stator has radial grooves, with outputs that have an open communication with the air gap; in each of the slots, at least one of the coils of the stator is fixed by an elastic retaining mechanism which is in contact with wedges which are slidably mounted in a symmetrical set of notches provided for this purpose in the stator in the output of each slot; This apparatus is characterized by comprising: a. a sensor dimensioned in such a way so that it can be inserted in the air gap, with a moving element, capable of being displaced transversally to this air gap by the effect of a pressure exerted by a fluid: b. a mechanism to position the sensor in front of the wedge of the stator whose voltage will be evaluated, in such a way that this mobile element faces the wedge of the stator; c. a mechanism for exerting a pressure by means of a fluid, to move the moving element of the sensor and cause this element to contact first and then press against the wedge of the stator, while the sensor is backed against the rotor; d. a mechanism for measuring the pressure exerted when the movable element presses against the wedge of the stator, this pressure being proportional to the force applied to the elastic retention mechanism that is located behind the wedge of the stator; and. a mechanism to measure the displacement of the wedge of the stator when the pressure is exerted; and f. a mechanism to determine the tension of the wedge of the stator, using the force and displacement measured in this way; in which: g. because the sensor has a body formed by a flat and thin plate, with a thickness slightly less than the width of the air gap; this plate has a surface with a recess that acts as a cylinder in which a piston is inserted that operates as the aforementioned moving element; this piston can be operated remotely, injecting the fluid under pressure in the cylinder; and h. The mechanism to measure the displacement of a wedge of the stator consists of a mechanism to measure the displacement of the piston in the cylinder; that piston displacement measuring mechanism consists of at least one elastic blade having a first end firmly fixed to the surface of the sensor; a second end mounted on the piston, so that it can be moved with it; and a central portion to which at least one strain gauge is attached giving a signal proportional to the displacement of the piston and therefore, to the displacement of the wedge of the stator when it is pressed. The apparatus of claim 12, characterized in that the mechanism (b) has a first mechanical system that allows the displacement of the sensor along each radial slot. 14. The apparatus of claim 13, characterized in that the first mechanical system is operated by a motor that is controlled by a control device, to automatically move the sensor along each radial slot, in which the sensor can be positioned in front of each successive wedge of the rotor, inserted slidably in the grooves of said radial groove. 15. The apparatus of claim 14, characterized in that the first mechanical system has a chain wound on a drum provided by the motor, and this chain has a free end that can be unwound within the air gap and to which the sensor is attached. . 16. The apparatus of claim 14, characterized in that the mechanism (b) also has a second mechanical system that allows the sensor to move from a given slot to at least one other adjacent slot. The apparatus of claim 16, characterized in that the second mechanical system has a carriage on which the first mechanical system is mounted, and mechanisms for moving the carriage during transport. 18. The apparatus of claim 14, characterized in that the first mechanical system is manually controlled and the sensor has a fiber display system that allows an operator to locate each wedge of the stator and position the sensor relative to it. The apparatus of claim 18, characterized in that the first mechanical system has a chain wound on a drum provided by the motor, and this chain has a free end that can be unwound within the air gap, and to which the chain is attached. sensor. The apparatus of claim 8, characterized in that the display system comprises an optical fiber having an end adapted to serve as a lens and is attached to the sensor. 21. The apparatus of claim 20, characterized in that the end of the optical fiber serving as a lens extends transverse to the piston in a cavity provided in the sensor body, and is opposite a mirror incorporated in that cavity and in which there is a lighting mechanism at the level of that sensor, to facilitate the visualization of a wedge of the stator, whose voltage will be valued, with the optical fiber. 22. The apparatus of claim 12, characterized in that the apparatus further has at least one separation plate that is attached to the body of the sensor, in the opposite place to the surface where the piston is mounted, by which the thickness of the sensor is increased. sensor, allowing its use in an air gap whose width is greater. 23. The apparatus of claim 12, characterized in that the mechanism (f) has an electronic conditioning system and a computerized processing system. 24. The apparatus of claim 23, characterized in that the fluid injected is compressed air and the apparatus is in the form of a portable device. SUMMARY OF THE INVENTION A method is described for quantitatively evaluating the tension of the stator wedges of an alternator that can be effected without having to remove or disassemble the rotor of an alternator, even though the alternator air gap is as thin as mm, and that allows to quantitatively evaluate the compression percentage of the spring for fluctuations that hold the wedges in a simple, effective and reliable way. This method uses a sensor in the form of a thin, flat plate that has a surface with a recess into which a piston is inserted. This sensor is inserted in the air gap of the alternator and positioned in front of the stator wedge to be evaluated, so that the piston faces the wedge. Then, a fluid is injected into the sensor for the piston to contact and then press the wedge, while the sensor is backed up against the rotor. The pressure of the injected fluid that is proportional to the force applied to the spring for fluctuations is measured, as well as the displacement of the stator wedge, while exerting the pressure, and the force and displacement measured in this manner are used to determine the tension of the wedge. This measurement of shifting of the wedge is achieved by measuring the displacement of the piston in its cylinder, by means of a strain gauge attached to elastic blades that bear against the piston. Also described is an apparatus for carrying out this method.